Page History

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This document focuses exclusively on the Maven 4.x core implementation (master branch of apache/maven). Specifically:

• The <sources> element introduced in POM model version 4.1.0
• The source/resource handling logic in DefaultProjectBuilder.java
• Validation gaps in the core that should be addressed

Compiler Plugin Compensation

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The goal should be to implement proper validation in the Maven core, providing early, consistent feedback with precise error locations via ModelProblem.

Origin of This Discussion

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<build>
  <sources>
    <source>
      <scope>main</scope>
      <lang>java</lang>
      <module>org.foo.moduleA</module>  <!-- Modular -->
    </source>
    <source>
      <scope>main</scope>
      <lang>java</lang>
      <!-- No module - classic style -->
    </source>
  </sources>
</build>

Current Workaround: The maven-compiler-plugin validates this at compile time and fails with:

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<build>
  <sources>
    <source>
      <scope>test</scope>
      <lang>java</lang>
      <module>org.foo.integrationTests</module>
    </source>
  </sources>
</build>

Root cause: The hasMain boolean remains false when no <source> has scope=main + lang=java. Line 697-698 then adds build.getSourceDirectory() (defaults to src/main/java from Super POM).

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<build>
  <sources>
    <source>
      <scope>main</scope>
      <lang>java</lang>
      <module>org.foo.moduleA</module>
    </source>
    <source>
      <scope>main</scope>
      <lang>java</lang>
      <module>org.foo.moduleB</module>
    </source>
  </sources>
</build>

Impact: Module-specific resources (like module-specific configs) cannot be organized per-module.

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<build>
  <sourceDirectory>src/custom/java</sourceDirectory>  <!-- User expects this to be used -->
  <sources>
    <source>
      <scope>main</scope>
      <lang>java</lang>
      <module>org.foo.bar</module>
    </source>
  </sources>
</build>

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Analysis

Core Design Question

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if (module specified) {
    // Modular mode
    path = directory specified ? baseDir.resolve(directory)
                               : baseDir/src/<module>/<scope>/<lang>
} else {
    // Classic mode
    path = directory specified ? baseDir.resolve(directory)
                               : baseDir/src/<scope>/<lang>
}

Strict vs Lenient Approach

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Configuration conflicts result in errors that fail the build.

Approach B: Lenient (Warn and Continue)

Configuration conflicts result in warnings but the build continues.

Comparison

Recommendation

Approach A (Strict) is recommended for new projects because:

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This matrix shows all configuration combinations and their expected behavior under each approach.

Legend

Java Sources

Resources

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Current Implementation Status

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// Lines 669-686: Iterate over <sources> and track what's present
boolean hasScript = false;
boolean hasMain = false;   // true if ANY <source> has lang=java, scope=main
boolean hasTest = false;   // true if ANY <source> has lang=java, scope=test
for (var source : sources) {
    var src = DefaultSourceRoot.fromModel(session, baseDir, outputDirectory, source);
    project.addSourceRoot(src);
    // ... tracking logic
}

// Lines 694-701: Decide whether to use legacy sourceDirectory/testSourceDirectory
// (Lines 687-692 contain a comment explaining this behavior)
if (!hasScript) {
    project.addScriptSourceRoot(build.getScriptSourceDirectory());
}
if (!hasMain) {
    project.addCompileSourceRoot(build.getSourceDirectory());  // Silent fallback
}
if (!hasTest) {
    project.addTestCompileSourceRoot(build.getTestSourceDirectory());  // Silent fallback
}

// Lines 703-708: Resources are ALWAYS added from legacy elements (no checks!)
for (Resource resource : project.getBuild().getDelegate().getResources()) {
    project.addSourceRoot(new DefaultSourceRoot(baseDir, ProjectScope.MAIN, resource));
}
for (Resource resource : project.getBuild().getDelegate().getTestResources()) {
    project.addSourceRoot(new DefaultSourceRoot(baseDir, ProjectScope.TEST, resource));
}

Gap Analysis

Key insight: Only the mixed modular/non-modular validation (#8) is currently caught by the compiler plugin. All other issues require core implementation.

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Implementation Approaches

Phase 1: Module-Aware Resource Handling (Implemented)

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Priority Hierarchy (Proposed):

Example - Would Work Without Explicit Plugin Configuration:

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1. Per-module tracking instead of project-level booleans:

   Code Block
   language java collapse false
   record ModuleConfig(boolean hasMain, boolean hasTest, boolean hasMainResources, boolean hasTestResources) {} Map<String, ModuleConfig> moduleConfigs = new HashMap<>();

2. Validation in ModelValidator (not DefaultProjectBuilder):

   • Move conflict detection to validateEffectiveModel()
   • Report ModelProblem with line numbers and severity
   • Early feedback during POM processing

3. Strict validation rules (from Section 2.2):

   Rule	Condition	Severity	Message
   R1	<sources> + explicit <sourceDirectory>	ERROR	"Cannot combine <sources> with explicit <sourceDirectory>"
   R2	Mixed modular/non-modular sources	ERROR	"Cannot mix modular and non-modular sources"
   R3	Duplicate <source> definition	ERROR	"Duplicate source for scope='X', lang='Y', module='Z'"
   R4	<sources> configured but classic dir exists	WARN	"Directory 'src/main/java' exists but will be ignored"

4. Fix test-only modular project injection (Problem 2):

   Code Block
   language java collapse false
   // Don't inject legacy main sources for modular projects if (!hasMain && !isModularProject) { project.addCompileSourceRoot(build.getSourceDirectory()); }

5. Warning when explicit configuration is ignored (Problem 4):

   Code Block
   language java collapse false
   if (hasMain && isExplicitSourceDirectory(build)) { // Report ModelProblem with line number }

6. InputLocation-based detection for explicit vs inherited configuration:

   Code Block
   language java collapse false
   InputLocation location = build.getLocation("sourceDirectory"); boolean isExplicit = location != null && !isSuperPomLocation(location);

Implementation Location:

Implementation Timeline

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Reference Material

Model Version vs Maven Version

The <sources> element requires model version 4.1.0:

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Where to Implement Validation

Related Code Locations

• impl/maven-impl/src/main/java/org/apache/maven/impl/model/DefaultModelValidator.java
• impl/maven-core/src/main/java/org/apache/maven/project/DefaultProjectBuilder.java
• impl/maven-impl/src/main/java/org/apache/maven/impl/DefaultSourceRoot.java
• impl/maven-impl/src/main/resources/org/apache/maven/model/pom-4.0.0.xml (Super POM 4.0)
• impl/maven-impl/src/main/resources/org/apache/maven/model/pom-4.1.0.xml (Super POM 4.1)

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